In a disc drive, a spindle motor assembly comprises a stationary portion and a rotary portion. The rotary portion, also referred to as the disc pack assembly, typically includes a number of discs and spacers mounted on a spindle hub. As the disc pack assembly is rotated, the lack of perfect symmetry about the axis of rotation results in vibration of the disc pack assembly. Therefore, it is necessary to balance the disc pack assembly.
For ease of reference, the end of the spindle hub which is mounted nearer a spindle base flange in the disc drive will be referred to as the bottom end, and the opposite end of the spindle hub will be referred to as the top end.
One method of balancing a disc pack assembly is described in the U.S. Pat. No. 5,555,144 issued Sep. 10, 1996, to Wood, et al. for “Balancing System for a Disc Drive Disc Assembly”. This method involves mounting a C-shaped balancing clip to the disc pack assembly such that, by selecting a balancing clip with an appropriately located center of mass and selecting an appropriate orientation for the balancing clip, the disc pack assembly can be balanced.
To achieve two-plane balance, the balance at the top end and at the bottom end of the disc pack assembly are separately adjusted by mounting balancing clips to both the top and the bottom ends of the spindle hub. In the case of an “under-hub” spindle motor assembly, the bottom end does not provide access for the balancing clips to be mounted because of the way the motor magnet and the stator are arranged below the spindle hub flange.
Another approach to balancing is described in the U.S. Pat. No. 5,621,588 issued Apr. 15, 1997 to Williams, et al. for “Two-Plane Balance for Hard Disk Drive Motor-Base Assemblies.” According to this method, a set of balancing screws are fastened to a top surface of the spindle hub, and another set of balancing screws are fastened to a bottom surface of the spindle hub. The spindle motor assembly is mounted on a base plate by means of mounting screws. The mounting screws are chosen to be larger than the balancing screws in order that the bottom surface of the spindle hub can be accessed from the bottom direction by removing a mounting screw.
Thus, it has been generally inconvenient and difficult to adjust the bottom-plane balance of a disc pack assembly. Therefore, there remains a need for an improved method of balancing a disc pack assembly. It will be evident from the following description that the present invention offers this and other advantages over the prior art.